Despite recent declines in high-flying growth stocks, bankers remain uniformly optimistic that the IPO window will be open for many quarters to come. But one important bellwether—the biotech sector—suggests that investor appetite for the riskiest startup companies may indeed be starting to wane.

Certainly 2014 so far has been a great time for venture investors and founders to cash out. The most recent tech IPOs, including GrubHub and Five9, have mostly done well despite the market tremors. The research firm CB Insights says that the technology IPO pipeline in the United States is nearly 600-companies strong.

But biotech, a quirky corner of the IPO world, has recently shown small cracks.